Stemming immigration to the United States was a central plank of Trump's election campaign.

His signature policy was to build a wall across the 2,000 mile (3,200 kilometer) border between the United States and Mexico.

Some of the border is already fenced, but Trump has argued that a wall is needed to stop illegal immigrants entering from Latin America.

Experts have voiced serious doubts about whether a wall would actually stem illegal immigration, or if its worth investing billions in such a project when there are other cost effective methods of achieving similar results.

But the issue has become a clarion call for the US right and far right, the core of Trump's support.

Trump sworn in as 45th US president

After four days in office some are wondering why the populist president's policies have been largely indistinguishable from those of orthodox Republicans.

His promised executive order may be a way to slake that thirst.

Still, any action from the White House would be piecemeal, diverting only existing funds toward the project.

The Republican-controlled Congress would need to supply new money if the project is to be anywhere near completed, and Trump's party has spent years preaching fiscal prudence.

Furthermore, much of the land needed to build the wall is privately owned, implying lengthy legal proceedings, political blowback, and substantial expropriation payments.

Homeland Security Secretary John Kelly even told his confirmation hearing that the border wall might not "be built anytime soon." Trump had promised to make "Mexico pay" for the wall, something that the Mexican government says is fanciful.

Trump aides have weighed increasing border tariffs or border transit costs as one way to "make Mexico pay." .

Trump has also floated the idea of a ban on Muslims coming to the United States.

He is said to be considering moves to halt or slow refugee flows, particularly from Syria and other Muslim-majority countries.

Around 4.8 million Syrians have fled to neighboring countries alone, according to the United Nations.

An estimated 18,000 Syrians have fled to the United States.

Former officials said Trump could slow the process down by move resources away from processing visa requests, or cut migrant quotas and programs.

The orders would restrict immigration and access to the United States for refugees and visa holders from Iraq, Iran, Libya, Somalia, Sudan, Syria and Yemen, according to the Washington Post.

Citizens from those countries already face large obstacles in obtaining US visas.

But the move has prompted a fierce backlash even before it was announced.

"Donald Trump is making good on the most shameful and discriminatory promises he made on the campaign trail," said Trita Parsi of the National Iranian American Council. Iran was one of the countries that may be listed.

"He called for a Muslim ban and is now taking the first steps to implement one. This will not stand. The American people are better than this." Trump may also issue orders on so-called sanctuary cities, where local officials refuse to cooperate with federal immigration authorities on such things as handing over illegal immigrants for deportation.